1. Field of the Invention
The present invention relates to a discrete multi-tone (DMT) data communications network, more particularly to a DMT transceiver device.
2. Description of the Related Art
Modern society continues to create exponentially increasing demands for digital information and the communication of such information between data devices. Local area networks use a network, cable or other media to link stations on the network for exchange of information in the form of packets of digital data. A typical local area network architecture uses a media access control (MAC) enabling network interface cards at each station to share access to the media. Most conventional local area network architectures use media access controllers operating according to half-duplex or full-duplex Ethernet (ANSI/IEEE standard 802.3) protocol and a prescribed network medium, such as twisted pair cable.
These architectures have proven quite successful in providing data communications in commercial applications. However, these common local area network architectures require installation of specialized wiring and use of specific wiring topologies. For example, the most popular network protocols, such as Ethernet, require special rules for the wiring, for example with regard to quality of wire, range of transmission and termination.
Due to the success of the Internet and the rapid decreases in the prices of personal computers and associated data equipment, a demand has arisen for data communications between a limited number of devices within relatively small premises, typically a residence or small business. While existing local area networks can serve the purpose, in such installations, the cost of installing physical network wiring satisfying the rules for the particular protocol can be prohibitively expensive.
Most existing buildings, including residences, include some existing wiring, for phones, electrical power and the like. Proposals have been made to communicate data using such existing infrastructure. This reduces the costs of wiring for the network, but the existing wiring raises a variety of issues regarding transport of high-speed digital signals.
For example, efforts are underway to develop an architecture that enables computers to be linked together using conventional twisted pair telephone lines. Such an arrangement, referred to herein as a home network environment, provides the advantage that existing telephone wiring in a home may be used to implement a home network environment without incurring costs for substantial new wiring installation. However, any such network must deal with issues relating to the specific nature of in-home telephone wiring, such as operation over a media shared with other services without interference from or interfering with the other services, irregular topology, and noise. With respect to the noise issue, every device on the telephone line may be a thermal noise source, and the wiring may act much like an antenna to pick up disruptive radio signal noise. Telephone lines are inherently noisy due to spurious noise caused by electrical devices in the home, for example dimmer switches, transformers of home appliances, etc. In addition, the twisted pair telephone lines suffer from turn-on transients due to on-hook and off-hook and noise pulses from the standard telephones coupled to the lines, and electrical systems such as heating and air conditioning systems, etc.
An additional problem in telephone wiring networks is that the signal condition (i.e., shape) of a transmitted waveform depends largely on the wiring topology. Numerous branch connections in the twisted pair telephone line medium, as well as the different associated lengths of the branch connections, may cause multiple signal reflections on a transmitted network signal. Telephone wiring topology may cause the network signal from one network station to have a peak-to-peak voltage on the order of 10 to 20 millivolts, whereas network signals from another network station may have a value on the order of one to two volts. Hence, the amplitude and shape of a received pulse may be so distorted that recovery of a transmit clock or transmit data from the received pulse becomes substantially difficult.
At the same time a number of XDSL technologies are being developed and are in early stages of deployment, for providing substantially higher rates of data communication over twisted pair telephone wiring of the telephone network. XDSL here is used as a generic term for a group of higher-rate digital subscriber line communication schemes capable of utilizing twisted pair wiring from an office or other terminal node of a telephone network to the subscriber premises. Examples under various stages of development include ADSL (Asymmetrical Digital Subscriber Line), HDSL (High data rate Digital Subscriber Line) and VDSL (Very high data rate Digital Subscriber Line).
Consider ADSL as a representative example. For an ADSL-based related service, the user""s telephone network carrier installs one ADSL modem unit at the network end of the user""s existing twisted-pair copper telephone wiring. Typically, this modem is installed in the serving central office or in the remote terminal of a digital loop carrier system. The user obtains a compatible ADSL modem and connects that modem to the customer premises end of the telephone wiring. The user""s computer connects to the modem. The central office modem is sometimes referred to as an ADSL Terminal Unitxe2x80x94Central Office or xe2x80x98ATU-Cxe2x80x99. The customer premises modem is sometimes referred to as an ADSL Terminal Unitxe2x80x94Remote or xe2x80x98ATU-Rxe2x80x99. The ADSL user""s normal telephone equipment also connects to the line through a frequency combiner/splitter, which is incorporated in the ATU-R. The normal telephone signals are split off at both ends of the line and processed in the normal manner.
For digital data communication purposes, the ATU-C and ATU-R modem units create at least two logical channels in the frequency spectrum above that used for the normal telephone traffic. One of these channels is a medium speed duplex channel; the other is a high-speed downstream only channel. Two techniques are under development for dividing the usable bandwidth of the telephone line to provide these channels. One approach uses Echo Cancellation. Currently, the most common approach is to divide the usable bandwidth of a twisted wire pair telephone line by frequency, that is to say by Frequency Division Multiplexing (FDM).
FDM uses one frequency band for upstream data and another frequency band for downstream data. The downstream path is then divided by time division multiplexing into one or more high-speed channels and one or more low speed channels. The upstream path also may be time-division multiplexed into corresponding low speed channels.
The FDM data transport for ADSL services utilizes discrete multi-tone (DMT) technology. A DMT signal is basically the sum of N independently QAM modulated signals, each carried over a distinct carrier frequency channel. The frequency separation between consecutive carriers is 4.3125 kHz with a total number of 256 carriers or tones (ANSI). An asymmetrical implementation of this 256 tone-carrier DMT coding scheme might use tones 32-255 to provide a downstream channel of approximately 1 MHz analog bandwidth. In such an implementation, tones 8-31 are used as carriers to provide an upstream channel of approximately 100 kHz analog bandwidth. Each tone is quadrature amplitude modulated (QAM) to carry up to 15 bits of data on each cycle of the tone waveform (symbol).
A conventional DMT system is shown in FIG. 6. The transmitter 601 includes a constellation point mapper 603 for logically mapping input bit streams onto a complex plane, whereby each sequence of bits (e.g., 2 bits) is equated to a complex number (i.e., constellation point). A constellation point represents the amplitude and phase of a particular tone. A typical ADSL system, for instance, employs 256 tones. An inverse Fast Fourier transform (IFFT) 605 then converts the constellation points, which provide information in the frequency-domain, to time-domain waveforms for transmission over the channel 625. Each conversion transforms 256 constellation points (complex numbers) into 512 samples of the time domain waveform. A parallel-to-serial block 607 clocks the samples out in a serial sequence for input to the analog front end (AFE) block 609, which is described below in the discussion of FIG. 7. The AFE block 609 outputs the actual bandpass waveform that is transmitted across the channel 625.
On the receiver side, the bandpass signal enters the receive AFE block 613. The AFE block 613 outputs a serial sequence of the digitized received waveforms to the input of the serial/parallel block 613, which converts the serial stream into a parallel set of data. The parallel data is then input into a Fast Fourier transform (FFT) 617 to extract the corresponding frequency-domain signals. The resulting frequency-domain data may display spectral power loss mainly because of the channel attenuation and digital to analog (D/A) conversion. Accordingly, the received signals usually undergo equalization to restore their spectral energy distributions. Slicer 621 then performs decoding of the complex numbers to corresponding bit streams.
FIG. 7 illustrates a traditional transmitter side AFE block 609 employed in the DMT system of FIG. 6. This transmitter side AFE block 609 comprises essentially four basic components: a D/A converter 701, low pass filter (LPF) 703, mixer 705, and a voltage controlled oscillator 707. The digital waveforms from the IFFT 605 are converted to an analog waveform (i.e., baseband signal). The baseband signal is fed into the LPF 703 to eliminate unwanted high frequencies; a typical cutoff frequency of the LPF 703 is 138 kHz. The filtered baseband signal is then up converted by mixer 705; the voltage controlled oscillator (VCO) 707 supplies a sinusoidal signal with an amplitude of A and frequency of co to the mixer 705. The mixer 705 and the VCO 707 operator as a modulator. Use of a VCO to directly frequency shift the modulating signal poses implementation constraints in terms of flexibility of integrated circuit (IC) design.
The AFE block 613 for the receiver side performs the above operations in essentially the reverse sequence. That is, the AFE block 613 receives the bandpass signal from the channel 625 and down converts it to restore the baseband signal. The baseband signal is then input to a LPF and then digitized with an analog to digital (A/D) converter (not shown).
The existing DSL systems provide effective high-speed data communications over twisted pair wiring between customer premises and corresponding network-side units, for example located at a central office of the telephone network. The DSL modem units overcome many of the problems involved in data communication over twisted pair wiring. However, for a number of reasons, the existing DSL units are not suitable to providing local area network type communications within a customer""s premises. For example, existing ADSL units are designed for point-to-point communication. That is to say, one ATU-R at the residence communicates with one ATU-C unit on the network end of the customer""s line, and the units are not useable for multi-point communications. Also, existing ADSL modems tend to be quite complex, and therefore are too expensive for in-home communications between multiple data devices of one customer. A need therefore still exists for techniques to adapt DMT type DSL communications for use in an in-home multi-point environment.
There exists a need for a DMT system that is tailored for use over existing in-home wiring. In particular, the DMT system needs to provide a technique for frequency shifting that is flexible in terms of design choice and can be readily implemented without complex logic or circuitry.
These and other needs are satisfied by the present invention, where a communication system includes a transmitter circuit that outputs a symbol represented by differentially encoded signals over a range of frequencies (or tones). The transmitter utilizes the system clock to frequency shift the differentially encoded signals for transmission across the channel.
According to one aspect of the present invention, a communication system for transmitting a bit stream comprises a transmitter circuit that generates a symbol. The symbol includes differentially encoded signals, in which each of the differentially encoded signals is mapped to one of a plurality of carrier frequencies based upon the bit stream. A receiver circuit receives the symbol and decodes the differentially encoded signals portion of the symbol to output the bit stream. Each of the received differentially encoded signals has a plurality of spectral images associated with the corresponding carrier frequency and its harmonics. A system clock is coupled to the transmitter circuit for frequency shifting the differentially encoded signals based upon the clock rate of the system clock. This arrangement advantageously permits an efficient technique of up converting the baseband symbol.
Another aspect of the present invention provides a method for transmitting a bit stream. The method comprises generating a symbol comprising differentially encoded signals based upon the bit stream via a plurality of carrier frequencies. The method also includes receiving the symbol and decoding the differentially encoded signals of the symbol to output the bit stream. Each of the received encoded signals has a plurality of spectral images associated with the corresponding carrier frequency and its harmonics. Further, the method includes frequency shifting the differentially encoded signals based upon a clock signal. The above method provides a cost-effective implementation of modulating the symbol.
Additional advantages and novel features of the invention will be set forth in part in the description which follows, and in part may become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.